The present invention relates to an apparatus for alarming a decrease in tire air-pressure and a method thereof. More particularly, it relates to an apparatus for alarming a decrease in tire air-pressure and a method thereof in which a decrease in internal pressure of an actually running tire can be reliably determined and an alarm can be generated accordingly based on rotational information of right and left tires of different types attached to a driving shaft, e.g. a new tire and a worn tire.
Determination of decompression based on a relative comparison of the number of revolutions of tires has conventionally been performed by storing ratios of the number of revolutions of each tire at the time of straight-ahead driving, corresponding to variations in the number of revolutions at the time of manufacturing tires and by correcting the measured numbers of revolutions of tires to determine decompression. However, while this method is based on a presupposition that the corrected values are not dependent on running speeds, relative speed differences that are dependent on speed are actually generated between driving wheels and following wheels owing to factors such as air resistance.
In other words, at the time of running at a high speed, driving force is acting on the driving wheels to overcome air resistance although the vehicle is running at a constant speed. The wheel speed difference between driving wheels and following wheels becomes larger than compared to the time at which the vehicle is running at a low speed.
However, while such changes are corrected by a function of speed in prior art methods, the whole theory is based on a presupposition that both tires of the driving wheels are of the same type.
For instance, in the case where right and left wheels at normal air pressure are of the same type (and their degrees of wear are also substantially identical), the relative speed ratio of the right and left tires with respect to the slip rate (driving force) of the tire is substantially constant, and the right and left wheel ratios of the following wheels and driving wheels are not affected by driving/braking as shown in FIGS. 15 and 16. Further, as shown in FIG. 17, in the case where a blowout of one of the tires of the following wheels has occurred with the degree of wear having progressed to some extent, and this is exchanged with a spare tire, the right and left wheel ratios of the following wheels will not be affected by the driving force. On the other hand, in the case where a blowout of one of the tires of the driving wheels has occurred and is exchanged with a spare tire so that the driving shaft bears tires of different degrees of wear, and the vehicle is running at a high speed, the driving wheels will of course be faster than the following wheels in terms of wheel speed, and the difference in degrees of wear between right and left driving wheels will further result in a right and left wheel ratio that is obviously sloped with respect to the slip rate as shown in FIG. 18, and a relative difference in wheel speed between right and left tires will be generated by the action of the driving force or braking force. Since such relative differences are not at all considered in prior art determining methods for decompression, they might affect determined values for decompression, resulting in erroneous alarm or in a failure to alarm. Thus, in the case where tires of different degrees of wear are attached to the driving shaft and the right and left wheel ratio is sloped with respect to the driving force, there exists a danger that erroneous alarm is released at the time of driving or braking although the air-pressure is normal.
On the other hand, a method is known in which relationships between front and rear wheel ratios and speeds are preliminarily stored and employed for correction at the time of actual running (refer to Japanese Unexamined Patent Publication No. 137512 /1995). However, this method is trying to correct front and rear wheel ratios at the time of actual running by preliminarily storing the phenomenon that the driving wheels rotate faster with an increase of the driving speed of the vehicle even at a constant driving speed, since driving force as to overcome the running resistance is required. Thus, if tires of driving wheels are intermediately exchanged, no correction of favorable accuracy can be performed.
The present invention has been made in view of these circumstances, and it is an object of the present invention to provide an apparatus for alarming a decrease in tire air-pressure and a method thereof with which erroneous alarm or failure to alarm can be prevented by correcting relative speed ratios that can be obtained from rotational information of right and left tires of different types of driving wheels or following wheels.